1. Field of the Invention
The present invention relates to a spark plug for an internal-combustion engine in which a noble metal tip is joined to an electrode which undergoes spark discharge.
2. Description of the Related Art
Conventionally, a spark plug has been used for ignition of an internal-combustion engine. A commonly-used spark plug includes a metal shell surrounding and holding an insulator therein, to which a center electrode is inserted in an axial hole, and a ground electrode, one end of which is welded to a front end of the metal shell and the other end of which is opposed to a front end of the center electrode so as to form a spark discharge gap. In the spark discharge gap, opposing surfaces between the center electrode and the ground electrode are provided with noble metal tips for improving spark consumption resistance.
As a material for the noble metal tip, conventionally, a Pt alloy is used which has a high melting point, excellent heat resistance and good oxidation resistance. In the noble metal tip made from the Pt alloy, consumption of the electrode caused by the spark discharge is less than that of an electrode tip made from a base metal such as a Ni alloy. Consequently good ignition performance can be maintained for a long period of time, and therefore the durable life is good. However, when the noble metal tip made from a Pt alloy is used in the corrosive atmosphere of an internal-combustion engine, if grain growth proceeds at a surface layer portion thereof, crystal grains may peel (drop) off due to cracks formed at the grain boundaries.
In order to suppress grain growth, the addition of Ir is known to be effective. However, Ir tends to volatilize by forming an oxide when subjected to high temperature. A noble metal tip having high durability has been proposed, in which a noble metal which is hardly oxidized and volatilized such as Rh is included in a Pt—Ir alloy to suppress the oxidation and volatilization of Ir, while grain growth of Pt is suppressed by the presence of Ir (see for example, JP-B-61-30014).
3. Problems to be Solved by the Invention
However, in recent years, high output technology of the internal-combustion engine has progressed. As a result, the temperature in the combustion chamber of an internal-combustion engine in which the spark plug is used has become higher than in the past. Although Pt has a high melting point, excellent heat resistance and good oxidation resistance, there is a concern relating to oxidation at today's higher temperatures. On the other hand, there is an additional concern that when Ir is volatized to generate many fine gaps on a surface layer portion, the noble metal tip becomes fragile. In order to increase the durability of the noble metal tip, an increase in Rh content might be contemplated. However, when the Rh content is increased, grain growth of the noble metal tip is thereby promoted which tends to form cracks in the grain boundaries.
This invention has been made for solving the above problems, and an object thereof is to provide a spark plug having a noble metal tip, which can improve durability even when used in a high temperature environment.